Spectral variability and iron line emission in the ASCA observations of the Seyfert 1 galaxy NGC 4051

We present the results of an extensive analysis of the ASCA AO2 observation of the Seyfert 1 galaxy NGC 4051. The target exhibits broadband [0.5-10 keV] variability by a factor similar to 8 on time scales similar to 10(4) s, with a typical doubling time similar to 500 s. The spectrum is characterized by a strong emission excess over the extrapolated power law at energies E less than or equal to 1 keV. Absorption edges due to ionized oxygen species O VII and O VIII are detected together with an emission-like feature at E similar to 0.93 keV. The O VII edge undergoes significant variability on a timescale as low as similar to 10(4) s, whilst no contemporary variability of the O VIII feature is detected. Typical variability time scales place constraints on the location and the density of the absorbing matter. In the self-consistent hypothesis of a high energy (E greater than or equal to 2.3 keV) power law reflected by an infinite plane-parallel cold slab, a photon index change (Delta Gamma = 0.4) has also been observed; a natural explanation can be found in the framework of non-thermal Comptonization models. The iron line is redshifted (centroid energy E similar to 6.1 keV) and broad (sigma > 0.2 keV); multicomponent structure is suggestive of emission from a relativistic accretion disk; however if the disk is not ionized a contribution by a molecular torus or an iron overabundance by a factor similar to 1.5 are required.